Hepatocellular carcinoma (HCC) is a leading cause of cancer mortality worldwide, and its incidence has been increasing in the US over the past decade. The high rate of mortality and frequency of recurrence, as well as the limited benefit of current therapies, underscore the urgent need for new therapeutic approaches. HCC is known to express tumor-specific antigens which are potential targets for immunotherapy; however, tumors often generate immunotolerance, thwarting effective immune responses. The underlying mechanisms of HCC-induced immunotolerance have been difficult to study in the absence of realistic animal models. Recently, we established a unique HCC murine model in our laboratory, in which hepatocytes reproducibly develop into discrete foci of HCC concomitantly with progressive hepatic fibrosis/cirrhosis in immunocompetent mice. This model approximates the process that commonly occurs in human beings, resulting in tumors that are histologically similar to human HCC. Using this model, we have demonstrated that the FDA-approved chemotherapeutic agent, sunitinib, prevents tumor antigen-specific immunotolerance and works with immunotherapy to induce regression of established HCCs. Successful therapy is associated with changes in activated STAT3, PI3K, FoxP3, and IL-10 as well as a reduction of regulatory T cells (Tregs) and myeloid-derived suppressive cells (MDSCs). Our findings indicate that sunitinib blocks the development of an immunosuppressive state within the liver, allowing effective T cell-based immunotherapy. Our unique murine model combined with sunitinib's inhibition of immunosuppression provides an ideal platform to elucidate the cellular and molecular mechanisms of tumor antigen-specific immunotolerance associated with HCC. In this application, we will test our central hypothesis that sunitinib treatment inhibits molecular pathways in tumor-bearing mice and leads to the activation of effector CD8 T cells by subverting the negative regulatory effects of distinct immune cell subsets. In aim 1, we will define the role of sunitinib in both direct and indirect modulation of immune cells including CD8 and CD4 T cells, DCs and Tregs in vivo. We will approach this question by comparing the phenotype, function and interaction of these cells in tumor-bearing mice with and without sunitinib treatment. In aim 2, we will define the role of STAT3 and p110 subunits in modulating diverse immune cells in vitro and in vivo, and will identify the other critical immune suppressive factors inhibited by sunitinib using high throughput technology. The ultimate goal of this proposal is to identify a method by which immunosuppression in HCC can be reversed and translate these findings for practical use in patients with HCC.